Software products released into the field typically have some number of residual bugs that either were not detected or could not have been detected during testing. This may be the result of flaws in the test cases themselves, assumptions made during the creation of test cases, or the infeasibility of testing the sheer number of possible configurations for a complex system. Testing approaches such as perpetual testing or continuous testing seek to continue to test these applications even after deployment, in hopes of finding any remaining flaws. In this paper, we present our initial work towards a testing methodology we call "in vivo testing", in which unit tests are continuously executed inside a running application in the deployment environment. In this novel approach, unit tests execute within the current state of the program (rather than by creating a clean slate) without affecting or altering that state. Our approach has been shown to reveal defects both in the applications of interest and in the unit tests themselves. It can also be used for detecting concurrency or robustness issues that may not have appeared in a testing lab. Here we describe the approach, the testing framework we have developed for Java applications, classes of bugs our approach can discover, and the results of experiments to measure the added overhead.